This invention relates to an irreversible indicator for detecting oxidising agents, and in particular an oxygen indicator, comprising at least one redox-sensitive dyestuff, at least one semiconductor material and at least one electron donor. The indicator is activated by exposure to light of about 200-400 nm. The invention also relates to a UV light detector.
Modified atmosphere packaging (MAP) is widely used to prolong the useful life of many oxygen-sensitive items, such as: food, beverages, works of art, pharmaceuticals, medical diagnostic kits and sterilised packages. MAP includes: vacuum packaging, reduced oxygen packaging, oxygen absorption packaging and nitrogen and carbon dioxide flushed packaging. The major use of MAP is found in the food industry where it is routinely employed to extend the shelf life of: breads, cookies, cakes, pastries, nuts and snacks, candies and confectioneries, coffee and tea, whole fat dry foods, processed, smoked and cured meats and fish, cheeses and dairy products, dried fruits and vegetables, spices and seasonings, flour and grain items, fresh and pre-cooked pasta and noodles, birdseed and pet and animal food. It follows that a cheap, oxygen indicator is a highly desirable feature for any MAPed product, especially with regard to quality assurance and tamper-proofing. A number of indicators for this purpose have been reported previously. These indicators fall into two major categories: colourimetric and fluorimetric indicators.
Most colourimetric indicators for strong oxidising agents comprise a redox-sensitive dyestuff (such as methylene blue), an alkaline substance (such as calcium hydroxide) and a strong reducing agent (such as glucose) rendered strongly reducing by the alkaline substance. In the absence of strong oxidising agents the reducing agent reduces the usually highly coloured oxidised form of the redox-sensitive indicator to its reduced, usually much less coloured form. For example, the oxidised form of methylene blue (which is blue) is reduced, by alkaline glucose, to leuco-methylene blue (which is colourless). Leuco-methylene blue is readily oxidised back to methylene blue by a strong oxidising agent, such as oxygen. Such indicators need to be stored under anaerobic conditions and are usually reversible in response to oxidising agents and are light sensitive and have a responsivity that is markedly affected by the presence of acidic gases, such as CO2 and SO2.
Although the following now only refers to oxygen as the oxidising agent it should be appreciated that any oxidising agent is also applicable.
U.S. Pat. Nos. 4,169,811, 4,526,752, 5,358,876, 5,483,819 and 5,583,047 are relevant examples of the prior art of colourimetric oxygen sensors.
Most fluorimetric oxygen indicators comprise one or more indicating dyes that fluoresce strongly under anaerobic conditions, but have a much-reduced fluorescence under aerobic conditions. These indicators rely on the quenching of the electronically excited state of the indicator dye (such as the ruthenium (II) tris 4,7-dimethyl-(1,10-phenanthroline) complex) by oxygen. The fluorescent indicator dyes are usually encapsulated in an oxygen permeable matrix, such as a polymer. Most fluorimetric oxygen indicators are reversible in response to oxygen.
U.S. Pat. Nos. 4,657,736, 5,407,829, and UK patent application GB 2132348A are relevant examples of the prior art.
Current colourimetric oxygen indicators have a number of problems that have prevented their extensive application in MAP. These problems include one or more of the following: a relatively high cost, short shelf-life, a response that is affected by the presence of carbon dioxide (a common MAP packaging gas) and the need for anaerobic storage and, preferably, handling when being applied. Most fluorimetric oxygen indicators also suffer from a high relative cost and have the added problem of luminescence detection, since, in general the fluorescence from a fluorimetric oxygen indicator is less amenable to detection by the human eye than the colour change associated with a colourimetric oxygen indicator. Thus, with colourimetric oxygen indicators, untrained personnel can be used to judge if a package is still anaerobic, whereas with the fluorimetric oxygen indicators often a luminescence-detection system is required. Prior art devices and methods for oxygen detection also suffer from a lack of reliability and, because of their inherent reversibility towards oxygen, insufficient protection against deliberate tampering and compromise in food microbial safety.
As noted above both colourimetric and fluorimetric oxygen indicators usually give a reversible response towards oxygen. However, the latter is not a desirable feature in MAPed foods since, if the integrity of an oxygen-free MAPed food package is compromised, and air is let in, microbial growth can be so rapid that all the oxygen that leaks into the package is metabolized after a short time. This is particularly true if the air leak is small. Thus, following the creation of a small air leak in a MAPed package, within a short time a reversible colourimetric or fluorimetric oxygen indicator present in the package will indicate (correctly) the absence of oxygen, despite the compromise in the integrity of the package with its concomitant deleterious effect on food safety (i.e. undesirable microbial growth).
It is an object of the present invention to obviate/mitigate at least one of the aforementioned problems.